The present invention deals with aqueous compositions containing dispersions of metal particles and their use in particular types of recording elements.
In recent times, nanoscale dispersions of metal particles, prepared by chemical reduction, have gained importance as major constituents of aqueous coating compositions used for preparing particular types of recording elements. More specifically the use of these coating compositions has been disclosed for the manufacturing of heat mode recording materials, magnetic materials and conductive layers, which will now be described in more detail.
Conventional photographic materials based on silver halide are used for a large variety of applications. In the past several proposals have been made for obtaining an imaging element that can be developed using only dry development steps. Examples of such non-conventional systems are photothermographic systems, e.g. 3M""s dry silver technology, systems based on photopolymerization, and imaging elements that are exposed by an image-wise distribution of heat. When this heat pattern is applied directly by means of a thermal head such elements are called thermographic materials. When the heat pattern is applied by the transformation of intense laser light into heat these elements are called heat mode materials. In a particular type of heat mode recording materials information is recorded by creating differences in reflection and/or in transmission on the recording layer. The recording layer has high optical density and absorbs radiation beams which impinge thereon. The conversion of radiation into heat brings about a local temperature rise, causing a thermal change such as evaporation or ablation to take place in the recording layer. As a result, the irradiated parts of the recording layer are totally or partially removed, and a difference in optical density is formed between the irradiated parts and the unirradiated parts (cf. U.S. Pat. Nos. 4,216,501, 4,233,626, 4,188,214 and 4,291,119 and British Pat. No. 2,026,346)
The recording layer of such heat mode recording materials is usually made of a metal. For instance, the recording layers consists of bismuth, applied by evaporation in vacuo, as disclosed a.o. in EP 0 384 041. A commercially available material manufactured according to the principles of cited EP 0 384 041 is MASTERTOOL MT8, registered trade name, marketed by Agfa-Gevaert N.V. A drawback of the method of preparation of a thin bismuth recording layer by vacuum deposition is the fact that this is a complicated, cumbersome and expensive process.
Therefore, in EP 0 875 889 an alternative process for applying a thin metal layer is described comprising the following steps:
(1) preparing an aqueous-medium containing ions of a metal,
(2) reducing said metal ions by a reducing agent thus forming metal particles,
(3) coating said aqueous medium containing said metal particles on said transparent support.
In a preferred embodiment the metal layer is again a bismuth layer. However such bismuth layers coated from an aqueous medium suffer in their turn from another drawback. Compared to bismuth layers prepared by vacuum deposition their sensitivity to laser light is lower. This is due to the presence of a higher degree of oxidized bismuth, and to the presence of ballast compounds in the layer such as a binder and additives improving stability, which to a certain degree hamper the formation of microspheres by the action of laser radiation.
The drawbacks cited above were overcome by the teachings of pending European Patent application No. 98202784 published as EP-A 0 980 764 on Feb. 23, 2000, wherein a heat mode recording element is disclosed comprising, in order,
(1) a transparent support optionally carrying a subbing layer,
(2) a thin metal layer coated from an aqueous medium,
(3) a protective layer or layer pack,
characterized in that said heat mode recording element contains hypophosphorous acid, or phosphorous acid, or a mixture of both.
In the preferred embodiment of the cited patent the binder in the bismuth layer is carboxymethylcellulose (CMC). The reason for this choice is the fact that very small amounts of CMC or other cellulose derivatives are required to keep the concentrated metal dispersion stable. For instance, the amount in gram of CMC needed per gram of bismuth may be as low as 0.015. These very small amount allows to concentrate the metal particles dispersion, preferably by ultrafiltration, without causing an excessive increase in viscosity which would hamper the ultrafiltration and the coating of the final composition. However, it will be clear that the addition of (hypo)phosphorous acid acidifies the composition. Below a pH of about 4.5 the carboxylate groups of CMC will become protonated. As a result the stabilizing binder action of the CMC will be lost and flocculation of the bismuth dispersion is likely to occur. Therefore the practical concentration range of adding (hypo)phosphorous acid is limited and insufficient. Moreover, in storing conditions of high humidity the layer acidifies even more due to the reaction of hypophosphorous acid with residual Bi3+ions. This leads again to flocculation in the layer and a strong decrease of Dmax. When one tries to incorporate the (hypo)phosphorous acid in the first layer of the protective layer pack (which can be an adhesive layer or a soft polymeric layer) other disadvantages occur. The presence of the (hypo)phosphorous acid is less effective and due to a certain degree of layer mixing during coating the eveness of the bismuth layer is slightly affected, resulting in loss of sensitivity and increase of Dmin.
In pending European patent application Appl. No. 98203868, which was published as EP-A 0 997 918 on May 3, 2000, a process is disclosed for the preparation of a magnetic layer, said process comprising the steps of:
(1) preparing an aqueous solution containing one or more type of metal ions including nickel ions,
(2) chemically reducing said one or more metal ions by means of a reducing agent thus forming an aqueous dispersion of metal particles including nickel,
(3) removing all superfluous ions from said aqueous dispersion by means of a washing step, preferably an ultrafiltration and/or diafiltration step, or be means of centrifugation,
(4) coating the resulting aqueous dispersion onto a support.
When it is the intention to prepare a semi-soft magnetic layer the metal ions are pure nickel ions. In the case of a soft magnetic layer the nickel ions are mixed with iron, cobalt ions, molybdene ions or mixtures of these. In order to be effective such a magnetic layer must show a rather high metal coverage, say about 15 g/m2 for nickel. This can be achieved by coating a dispersion of 150 g nickel per liter at a wet thickness of 100 xcexcm. This requires a rather high viscosity for the coating composition in the order of 50 mPa.s. Therefore the addition of a viscosity regulating compound would be useful such as polyacrylic acid which causes a viscosity dependent on pH adjustment. Of course, again a binder must be used which stabilizes the aqueous coating composition under acid conditions. This is not the case when CMC is the binder as it is the case in the preferred embodiment of the cited disclosure.
In pending European patent application Appl. No. 99200554, published as EP-A 1 031 642 on Aug. 30, 2000, a process is disclosed for the preparation of a metal oxide based conductive layer, said process comprising the following steps, in order:
(a) preparing an aqueous medium containing at least one type of metal salt,
(b) chemically reducing said metal salt by a reducing agent to form a dispersion of metal particles,
(c) washing said dispersion of metal particles,
(d) coating said washed dispersion onto a substrate, thereby obtaining a coated layer containing metal particles,
(e) subjecting said coated layer to an oxidizing treatment to form a conductive layer containing metal oxide particles.
In a most preferred embodiment the metal particles are tin particles.
It was meanwhile found by us that the oxidizing treatment is pH dependent and could be enhanced or performed at a lower temperature when an acid oxidizing agent such as boric acid (H3BO3) is added to the Sn0 dispersion before coating. Again this requires the presence of an acid stable binder which is not revealed in the cited reference.
It is the object of the present invention to provide an aqueous coating composition comprising as major constituent a dispersion of metal or metal alloy particles, prepared by chemical reduction in aqueous medium, with improved colloidal stability under acid pH conditions.
It is a further object of the present invention to provide an improved heat mode recording element based on a thin metal layer coated from such an aqueous composition.
It is still a further object of the present invention to provide an improved magnetic recording element based on a thin metal layer coated from such an aqueous composition.
It is still a further object of the present invention to provide an improved metal oxide based conductive layer obtained by an oxidizing treatment of a layer coated from such an aqueous composition.
The objects of the present invention are realized by providing an aqueous composition, being stable under acid pH conditions, and comprising as major constituent a dispersion of metal or metal alloy particles, prepared by chemical reduction of metal ions in aqueous medium, characterized in that (1) said dispersion has a concentration of at least 20 g of particles per liter, (2) said particles show an average size between 5 and 200 nm, and (3) said aqueous composition further contains a N-quaternized cellulose as binder. The most preferred binder is a N-quaternized cellulose derivative having the following Chemical Abstract Index Name: cellulose, 2-hydroxyethylether, polymer with N,N,-dimethyl-N-2-propenyl-2-propen-1-aminium chloride (CA Registry Number 92183-41-0).
The scope of the present invention further encompasses a sheet or web material comprising a support and a metal layer coated from an aqueous composition as defined above. According to the metal chosen they may be used as heat mode recording material, as soft or semi-soft magnetic element, or as conductive layer after a special oxidizing treatment.
For the preparation of a heat mode recording material the preferred metal particles in the aqueous coating solution are bismuth particles. For the application as magnetic layer the metal is nickel or a mixture of nickel with one or more other metals. When the purpose is to prepare a metal oxide based conductive layer the preferred metal is tin optionally mixed with one or more other metals.